Enantiomer separation is a field which has been expanding for about twenty years both on the preparative and on the analytical levels. This is particularly true in the pharmaceutical field, where the law requires the separate study of optical isomers of any chiral component of a medication composition. Substituted polysaccharides have been the subject of a number of studies, and celuloses physically deposited on a silica gel support are commercially available. Such compounds have the disadvantage, however, of usually being soluble in polar organic solvents, which drastically limits their applications.
Recent solutions to the problem of solubility have been found by forming covalent bonds between the substituted polysaccharide and the support. Kimata et al. have published their results (“Analytical methods and instrumentation”, vol. 1, 23–29 (1993)) on a stationary chiral phase based on -tris-2,3,6(4-vinylbenzoate) cellulose deposited on silica gel, then polymerised on the support.
Chromatographic data obtained with two racemic test mixtures were as follows:
Deposited and polymerisedDeposited supportsupport1-(1-naphthyl1-(1-naphthylStilbene oxideethanol)Stilbene oxideethanol)k′11.082.151.041.47k′21.662.841.441.80α1.541.321.391.22RS3.632.343.821.44where:                k′1 and k′2 are partition ratios, i.e., if i=1 or 2,        
            k      ’        i    =                    t        Ri            -              t        0                    t      0                                          where tRi is the retention time of compound i;            and t0 is the non-retained solute transit time;                        α is the relative retention ratio:        
  α  =                              t          R2                -                  t          0                                      t          R1                -                  t          0                      =                            k          ’                ⁢        2                              k          ’                ⁢        1                            RS is the peak resolution:        
      R    S    =            1      4        ⁢          (                        α          -          1                α            )        ⁢          (                                    k            ’                    ⁢          2                                                    1              +              k                        ’                    ⁢          2                    )        ⁢                  (        N        )                    1        /        2                                                where N is the plate number                        
  N  =            a      ⁡              (                              t            R                    ω                )              2                                      where ω is the peak width at a given ordinate, related to the square of the standard deviation or variance σ2 by the relationship ω2=aσ2, giving                        
  N  =            16      ⁢                        (                                    t              R                        ω                    )                2              =          5.54      ⁢                        (                                    t              R                        σ                    )                2            
A systematic reduction in the relative retention ratios obtained can be seen between the deposited support and the deposited and polymerised support: 10% less on the trans-stilbene oxide (α varies between 1.54 and 1.39) and 25% less for the 1-(1-naphthyl)ethanol.
This phenomenon can be explained by partial solubility of the polymerised support due to incomplete polymerisation because of weak reactivity of the vinyl benzoate group under the reaction conditions used.
Kimata et al. did not describe any examples of separation in a pure polar solvent.
Okamoto et al. (in European patent EP-B-0 155 637) described polymers which are chemically bonded to a silica gel. In particular, they described grafting tris-2,3,6-phenylcarbamate cellulose onto silica gel via a tritylated intermediate, then forming a covalent bond between the silica gel and the partially derived polysaccharide carbamate, by the action of a diisocyanate.
The results of elemental analyses carried out during the different stages of synthesis were as follows (EP-B-0 155 637, page 8 to page 9, line 33).
C %H %N %1.Trityl cellulose deposited on silica15.401.230.092.Detritylated cellulose deposited on silica 3.610.60—3.Cellulose bonded to silica by toluene-2,4-———diisocyanate4.Cellulose phenyl carbamate bonded to silica 3.230.270.45and washed with THF/chloroform
The drop in the degree of grafting between the cellulose deposited on silica (2) and cellulose phenylcarbamate bonded to silica (4) is important knowing that the degree of (4) calculated after (2) is of the order of 14% of carbon. The loss of hydrocarbon moieties can thus be estimated to be 80% from formation of the covalent bond between the cellulose and the silica by the diisocyanate arm, followed by derivative formation by reacting the OH groups with phenyl isocyanate and final washing with chloroform.
No example of separation in polar solvents was given for the support obtained.
Okamoto et al (Japanese patent JP 06-206-893) have described an oligosaccharide chemically bonded to silica gel by means of an imine function reduced to an amine. Amylose is then chemicoenzymatically regenerated from this oligosaccharide. The available hydroxyl functions are then reacted with carbamate functions to form derivatives. No example of separation in a polar solvent was given.
It is important to use a large column excess for preparative applications. The possibility of using 100% of chiral material in the form of pure polymer beads of substituted polysaccharides instead of physically depositing them on a support has proved effective in increasing mass yields in preparative chiral chromatographic processes. Thus patents EP-B-0 348 352, EP-B-0 316 270 and International patent application WO 96/27639 relate to the production of cellulose beads for separating optical isomers.
However, pure polymer beads are soluble in polar solvents such as halogenated solvents—tetrahydrofuran, dioxane, etc. It is thus impossible to use these solvents either pure or in mixtures with high proportions of these solvents, to carry out isomer separation.
In order to overcome this disadvantage, Francotte et al. recommended irradiation polymerisation of polysaccharide derivatives. (WO 96/27615).
However, the degree of polymerisation appears to be difficult to control in such a process. No example of separation in a pure polar solvent is given.
Minguillon et al. described the synthesis of cellulose carbamates with partial derivatives formed by reaction with an undecenoyl chloride. However, the structure of the support was not explained (J. of Chromatog. A 728 (1996), 407–414 and 415–422).
Lange (U.S. Pat. No. 5,274,167) described the polymerisation of optically active methacrylic acid derivatives, but the structure of the support was not explained. No example of separation in a pure polar solvent was given.
The present invention concerns the preparation of novel chiral compounds and their use in preparing or separating enantiomers, in particular on a support or in polymer beads.